The invention relates to a double layer cathode for molten carbonate fuel cells and to a method for producing the same.
Currently, cathodes made of porous nickel oxide which are doped with lithium are used for molten carbonate fuel cells. Because the electric resistance of nickel oxide is relatively low (0.05 Ohmxc3x97cm) and its electrocatalytic activity is high (exchange current density 0.8 mA/cm2), such cathodes made of nickel oxide result in a good cell output. In addition, because of the low resistance, the cathode can be produced to be sufficiently thickxe2x80x94approximately 1 mmxe2x80x94, so that a uniform gas distribution is ensured. However, a disadvantage of cathodes made of nickel oxide consists of the fact that they do not have a sufficient resistance to corrosion.
In the case of cathodes made of nickel oxide, nickel oxide is dissolved on the electrolyte matrix produced of an LiAlO2 carrier material and an electrolyte material, on which the electrolyte matrix, the cathode and the anode of the fuel cell are arranged. Nickel is transported into the electrolyte matrix and is deposited there as metallic nickel. In the course of time, this leads to an internal short circuit of the cell. The dissolving and depositing rate of the nickel oxide amounts to 2 to 4 micrograms per hour per square centimeter, whereby the useful life of the fuel cell is limited to approximately 10,000 hours. However, a useful life of at least 40,000 hours is a prerequisite for an economical utilization of the fuel cell technology.
Since, in addition, the solubility of the nickel oxide in the electrolyte material of the electrolyte matrix increases linearly with a rising carbon dioxide partial pressure, the economically particularly interesting pressure operation is excluded in the case of molten carbonate fuel cells with cathodes made of nickel oxide.
Furthermore, lithium cobaltite/nickel oxide double layer cathodes are also known which are arranged in the fuel cell such that the lithium cobaltite layer faces the electrolyte matrix and the nickel oxide layer faces away from it. Because of the insertion of the lithium cobaltite layer, these cathodes permit the lowering of the nickel depositing rate in the electrolyte matrix to less than 0.4 micrograms per hour per square centimeter and thus extending the useful life of the molten carbonate fuel cell to the above-mentioned required value. However, the disadvantage of such conventional lithium cobaltite/nickel oxide double layer cathodes is an increased temperature dependence of the polarization resistance which, in the case of a practical operation of molten carbonate fuel cells, contains temperature fluctuations between approximately 600xc2x0 C. to 680xc2x0 C., causes considerable fluctuations in the fuel cell output. In addition, by means of this type of double layer cathode, it is difficult to lower the average operating temperature of the fuel cells to a temperature of less than 650xc2x0 C. because then the output or the efficiency will fall to such an extent that no effective heat-guided operation can be carried out. However, specifically the lowering of the medium operating temperature is necessary in order to reduce the hot corrosion of the current collectors (bipolar plates) manufactured of steel such that, by means of the entire fuel cell stack containing a larger number of fuel cells, for example, 100 individual cells, the desired operating time of 40,000 hours can be achieved.
From German Patent Document DE 44 14 696 A1, double layer cathodes are known which have one layer on the basis of cobalt and one layer made of nickel oxide.
German Patent document DE 42 41 266 C1 shows cathode materials which are produced by the mixed precipitation of cobalt salts and alkaline earth salts, in which case mixed oxides are formed and an addition of powderized lithium oxide takes place with a subsequent sintering.
The abstract of the Japanese patent application from Derwent concerning Japanese Patent Document JP 09092294 A indicates that two-layer electrodes are known which have a first layer of cobalt oxide and ceroxide and a second layer of nickel.
From the abstract of the Japanese patent application in Derwent concerning Japanese Patent Document JP 05266892 A, electrode materials are known which are the result of the thermal treatment of mixtures of hydrous suspensions containing cobalt solutions and ceroxide.
It is therefore an object of the invention to provide a cathode for a molten carbonate fuel cell which has a longer useful life and a lower dependence on temperature.
According to the invention, it is provided that, in the case of such a process, a first cathode layer is formed from a first cathode material; a second cathode material is produced in that cobalt oxide is activated by means of a co-precipitation with cerium and is treated with lithium carbonate to form a suspension; the suspension of the second cathode material is applied as the second cathode layer onto the first cathode layer and is dried; and the formation produced from the two cathode layers is sintered at a raised temperature.
Thus, by means of the invention, a double layer cathode for molten carbonate fuel cells which is catalytically activated by means of cerium is created which has a significantly prolonged useful life. It is an important advantage of the double layer cathode produced according to the process of the invention that its polarization resistance has a lower temperature dependence and the fuel cell therefore has a higher output also at an operating temperature lowered to below 650xc2x0 C. than in the case of a conventional double layer cathode.
The material of the first cathode layer advantageously consists of nickel.
For producing the second cathode material, lithium carbonate is advantageously added in a stoichiometric quantity to the cobalt oxide.
The suspension of the second cathode material is advantageously converted to lithium cobaltite during the sintering.
The sintering advantageously takes place at a temperature of between 500 and 700xc2x0 C., more preferably at a temperature of between 550 and 650xc2x0 C., and most preferably at a temperature of 600xc2x0 C.
The suspension of the second cathode material is advantageously applied in a layer of a thickness of from 50 to 200 xcexcm. The application of a layer of a thickness of between 80 and 150 xcexcm is particularly advantageous.
According to a further development of the process according to the invention, it is provided that the co-precipitation of the cobalt oxide takes place by the mixed precipitation of cerium(III) nitrate/zirconyl nitrate/yttrium nitrate/cobalt nitrate solution. This permits an activation with cerium zirconium yttrium mixed oxide in the lithium cobaltite layer, whereby a further reduction is achieved of the absolute value of the polarization resistance of the cathode produced according to the process of the invention.
In the case of the double layer cathode produced according to the process of the invention, it is particularly advantageous that, when the fuel cells assembled in an operationally ready manner with the double layer cathodes are started, the cobalt contained in the second material is oxidized to form cobalt oxide and reacts with the lithium carbonate to form lithium cobaltite doped with ceroxide, and the nickel of the first cathode material is oxidized to nickel oxide and is lithium-treated. Thus, when the fuel cells are started, the cathode blank produced by means of the process according to the invention is brought into its final form.
Furthermore, the object is achieved according to the invention by a double layer cathode for molten carbonate fuel cells which is characterized in that the cathode contains a first layer consisting of a first cathode material and a second layer consisting of cerium-activated lithium cobaltite. The advantage of the double layer cathode according to the invention is a longer useful life than that of conventional cathodes for melting carbonate fuel cells and a lower temperature dependance of the polarization resistance with respect to conventional lithium cobaltite nickel oxide double layer cathodes.
The first cathode material preferably is lithium-treated nickel oxide.
According to a further development of the invention, it is provided that the cerium-activated lithium cobaltite layer of the double layer cathode according to the invention is activated with cerium/zirconium/yttrium/mixed oxide. Its advantage is a further reduction of the absolute value of the polarization resistance of the double layer cathode according to the invention.
In the following, embodiments of the invention will be explained by means of the drawing.